Simple
materialism, in itself, is patently wrong, simply because we are now
aware of all manner of phenomena (even material) that go beyond the
reach of the five senses: from radio waves, to cosmic rays, to galaxies
beyond the reach of our eyes, to chemical reactions and particle interactions
too small to be seen or felt; and that go beyond our current "common
sense" notions of reality ("the earth is flat", "the
sun goes around the earth", etc.)

Scientific
materialism augments materialism in a specific way: reality
also includes whatever is directly perceivable via (or directly inferable
from) scientific instrumentation used in carefully controlled experiments.
Thus even though the immediate senses cannot detect radioactivity, scientific
instrumentation can, and thus radioactivity is also considered “real”
by scientific materialists. Just so, the reality of galaxies beyond
the reach of the visible, or particles or individual molecules, etc.
In order for an instrument to classify as “scientific” (and thus be
usable for helping to extend the known reality), the instrument must
objectify; that is, there is the
perceiver, and there is what is perceived via the instrument (whether
a telescope or the five senses), and the clear distinction between the
two is what allows for “objective” results.

By extension, Freud’s
“unconscious”, too, falls within the purview of what scientific materialists
(like Freud) consider real. The “tools” used by the psychotherapist
include analysis of dreams, association games, recording of the behavior
patterns of patients in relation to their mother and father, and to
men and women in general, etc. Just as the doorbell ringing commonsensically
indicates someone (or something) on the other side of the door, so all
the data collected by these psychoanalytic tools suggest that there
is something “on the other side” of the conscious driving a human being,
apart from their conscious intentions and their autonomic nervous system.

What that “something”
—
the "unconscious" —
is altogether, is another matter.